In the microelectronics industry, there is a continued desire to reduce the size of structural features and/or to provide greater amount of circuitry for a given chip size. Advanced lithographic techniques are required to fabricate high performance and high density circuitry. Lithography impacts the manufacture of microscopic structures not only in terms of directly imaging patterns on the desired substrate, but also in terms of making masks typically used in such imaging.
Over approximately the last 20 years, the industry has migrated to shorter wavelength photolithography as the primary means of scaling the resolution to sustain the progressive demand for smaller features. The wavelength of imaging radiation used in photolithography has migrated from mid-ultraviolet (MUV) wavelengths (350-450 nm) to deep-UV (DUV) radiation (190-300 nm) and toward vacuum UV (VUV, 125-160 nm). Likewise the radiation-sensitive resist materials used in photolithography have evolved. MUV lithography employed diazonaphthoquinone (DNQ) and novolac-based resists. These materials offered high performance but were not extendible to DUV and VUV wavelengths due to their opacity at these shorter wavelengths. In addition, these resists were not of sufficient sensitivity to afford high throughput manufacturing.
Chemically amplified resists (CARs) were developed in response to the need for new resist materials for use in DUV photolithography. For positive tone CARs, labile moieties of the polymer are cleaved by acid-catalyzed thermolysis reaction (the acid-catalyzed reaction using photochemically-generated acid from a radiation-sensitive acid generator) that renders the resulting (deprotected) form of the polymer soluble in a subsequently applied developer, such as aqueous base. Thus, an image of the projected patternwise radiation is formed in the resist film after development, which can then serve as an etch-resistant mask for subsequent pattern transfer steps. The resolution obtained is dependent on quality of aerial image and ability of resist to maintain that image.
One barrier to imaging in the sub-50 nm half-pitch regime is a phenomenon known as image blur which diminishes the integrity of the pattern. Image blur can be defined as the deviation of the developable image from that of projected aerial image which is transferred into the film as the concentration of photochemically generated acid. While accelerating the rate of the deprotection reaction, the application of thermal energy diminishes the fidelity of the aerial image of acid formed during the patternwise exposure. Image blur can be divided into two contributing factors: gradient-driven acid diffusion and reaction propagation. Both factors contribute to blur, but to different degrees and with different temperature dependence. Both of these contributing factors can be tempered by the addition of acid-quenchers, or bases, which have been shown to reduce image blur. Appropriate baking conditions can optimize the resolution attainable with CARs. However, these approaches only reduce the image blur to low blur imaging.
For E-beam lithography (where an electron beam is used as the imaging radiation), currently there is a desire to image sub-50 nm features in the context of direct write applications and possibly in the future for mask-making where the imaging radiation is imaging even finer features (e.g., in the case of extreme UV imaging). At such scale, blur is especially problematic.
Non-chemically amplified (non-CA) resists and low activation (energy) protecting group resists have been proposed to achieve high resolution, low blur. Conventional DNQ systems are non-CA resists which have poor performance in E-beam lithography. Poly(methyl methacrylate) (PMMA) is a well known non-CA resist which undergoes main chain scission with a low quantum yield of 0.04 to 0.14, depending on the intensity and wavelength of the radiation. Side chain scission with a minute quantum yield of 10−6 has also been reported, but is too inefficient for practical resist applications. PMMA polymer also requires solvent as developer. Another well known non-CA E-beam resists are based on polybutene sulfone, however, these resists are not etch resistant (for reactive ion etching (RIE)), and thus the pattern cannot be transferred by a RIE process.
Another approach to improved resolution involves the use of multilayer (e.g., bilayer) resist systems where the resist layer itself is thinned to enhance resolution. The typical imaging resists for bilayer lithography contain silicon. Silicon-containing CA resists developed for short wavelength (e.g., 157 nm or greater) imaging are described in US patent publication Nos. 20020090572 and 20020081520. Multilayer resist systems are generally more complex and expensive to implement. It is not apparent that these systems would be usable with e-beam radiation or would be free of blur problems.
Thus, there is a need for improved resists for sub-50 nm lithography, especially for e-beam lithography. There is especially a need for resist compositions useful in e-beam lithography which resists provide good etch resistance toward oxygen plasma, good resolution, reasonable sensitivity to imaging radiation and minimal vulnerability to airborne base contamination.